Projections are exercises showing the possible future size of a population and its age and sex distribution. In making projections, demographers make assumptions about future trends in FERTILITY,MORTALITY, and MIGRATION. Because projections can vary significantly depending on their assumptions, users of projections need to understand these assumptions before using them. Projections alert policymakers and program planners to major demographic trends that will affect key social and economic programs. The three main projections produced by the UN Population Division (UNPD) illustrate how assumptions can produce significantly different results over time.

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FERTILITY VARIANTS

The Medium Fertility Variant, most often cited, assumes an increase in the use of family planning resulting in fertility reductions in patterns similar to what occurred in other countries. The Low Fertility Variant simply assumes that in each country the average number of children a woman would bear at most periods in time is one-half child less than the Medium Fertility Variant, while the High Variant assumes that figure is one-half child more than the Medium Variant. Under these three scenarios, world population total would range from 8.3 billion to 10.9 billion in 2050 and from 6.8 billion to 16.6 billion in 2100.

The majority of developing countries still have fertility above replacement level. Under the UNPD’s three scenarios, assumed fertility in Kenya in 2050 would range from 2.2 to 3.2 children per woman—down from 4.6 in 2010. Kenya’s population total in 2050 would range from 85 million to 110 million. A common misunderstanding is that fertility declines are automatic and continuous, as projections assume. Declines in fertility, however, typically depend on increased investments in family planning services, health, and education—particularly that of women and girls. Many countries that have not adequately invested in these areas have not experienced the fertility declines assumed in past projections and have had projections continuously revised upward. In other cases, countries that have invested adequately have seen fertility decline more rapidly than originally assumed and projections have been revised downward.

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One common misunderstanding about population projections is that when fertility declines to replacement level, population will immediately stop growing. In countries that previously had high fertility, however, population will continue to grow for many decades after fertility reaches replacement. Years of high fertility result in a “young” population, which generates momentum for future growth as the number of young people born when birth rates were high begin having children of their own. Brazil had its fertility decline to below replacement in the mid-2000s, but the UNPD projects its population to continue its growth until mid-century. Also, population often continues to grow even after the last year of a projection. Years of below replacement fertility result in an aged population, or a large proportion of elderly. This is now seen in most developed and many developing countries.

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FERTILITY ASSUMPTIONS AND POPULATION PROJECTIONS

FERTILITY ASSUMPTIONS AND POPULATION PROJECTIONS

FERTILITY ASSUMPTIONS AND POPULATION PROJECTIONS

Fertility often has the largest effect on future population size. Fertility is expressed as the total fertility rate (TFR), a measure of how many children women would bear in their lives if the rate of childbearing of a given year remained unchanged. Where fertility is high, it is usually assumed that fertility will decline as it did in countries with low fertility and eventually stabilize near two children per woman. That is referred to as “replacement level” fertility as couples merely “replace” themselves. Because of great variability in the future course of fertility, multiple projections are often created using different fertility scenarios, such as the UNPD’s High, Medium, and Low variants.

The HIV epidemic and its substantial impact on mortality in countries with high prevalence created the need to consider the future course of HIV infection as well as its treatment in mortality assumptions. In those countries with expanding HIV epidemics during the 1990s, death rate assumptions were revised upward in projections. Despite the rise in mortality, population growth continued, albeit at a slower rate due to the impact of HIV. Malawi’s population projections for 2050 and 2100 are 50 million and 130 million—these are 8 million and 23 million lower than the projections without the impact of HIV. The impact of HIV on mortality has, however, begun to decline. The UNPD assumes that mortality from HIV will continue to decline due to ever-increasing access to antiretroviral therapy and fewer new infections.

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For many developed countries, declining mortality at older ages combined with very low fertility is of considerable interest because of their impact on population aging. Japan’s life expectancy, the world’s highest today, is projected to increase from 84 years at present to 89 years in 2050 and 94 years by the end of the century. Projected increases in life expectancy at age 60 are equally dramatic: a 60-year-old Japanese can expect to live, on average, 26 more years today, 31 years in 2050, and 35 years at the end of the century. Due to increasing life expectancies at older ages and very low fertility, projections for many developed countries show the proportion of the population ages 65 and over rising as high as 30% to 40% by mid-century, an unprecedented development.

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LIFE EXPECTANCY AT BIRTH AND AT AGE 60

TOTAL POPULATION WITH AND WITHOUT THE EFFECT OF HIV

Mortality is incorporated into projections by estimating death rates by age groups and sex. Where mortality is relatively high and the resulting life expectancy at birth relatively low, changes in mortality also play an important role in future population size. Where mortality is already low and life expectancy has risen to high levels, mortality has much less effect. Demographers generally assume a continued decline in death rates and increased life expectancy. For the countries affected by the HIV epidemic, the impact of HIV on mortality is also assumed to continue for many decades, though now at a declining rate.

BETWEEN MORE AND LESS DEVELOPED REGIONS

International migration can be particularly unpredictable and difficult to incorporate into projection assumptions. Migration flows often result from short-term and unanticipated changes in economic, social, environmental, or political factors. For many countries, reliable information on the number of immigrants and emigrants is not available. Given those factors, it may often be assumed that current migration levels will persist for a time and then slowly decline. Migration assumptions often take into account countries with historically high immigration, such as the United States. Still, the UNPD projects that net flows of immigrants from developing to developed regions will decline from today’s 2.6 million annually to 2.1 million in 2050, finally reaching zero by 2100.

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SOURCES:

United Nations Population Division, World Population Prospects: The 2012 Revision.

United Nations Population Division, World Population Prospects: The 2010 Revision.